The present disclosure relates to an image forming apparatus for performing a fixing processing onto a sheet carrying a transferred image by an induction heating (IH) system.
A fixing device in an image forming apparatus is provided with a fixing roller and a pressing roller. A sheet carrying a transferred toner image is allowed to pass a fixing nip portion formed by the fixing roller and the pressing roller. After having passed the fixing nip portion, the sheet undergoes heating and pressurization to thereby fix the toner image onto the sheet. As such a fixing device, there is known an IH fixing device employing an induction heating system as a system for generating heat to apply the heat to a sheet. The IH fixing device includes an IH unit provided with an induction heating coil for generating magnetic flux and a core member for forming a magnetic path, and a fixing unit having a heating roller (or a heating belt) to be inductively heated by the IH unit.
The fixing unit is required to be exchanged resulting from e.g. aging deterioration. In this case, it is uneconomical to exchange the entirety of the IH fixing device regardless that the IH unit is not deteriorated. The IH unit and the fixing unit are configured individually. Specifically, the IH unit is mounted on a main body of the image forming apparatus, and the fixing unit is detachably mounted on the apparatus body. In the case where these units are configured individually, it may be difficult to keep the distance between an induction heating coil and a heating roller unchanged before the fixing unit is dismounted and after the fixing unit is mounted. If the distance between the induction heating coil and the heating roller varies, the induction heating effect also varies, which makes it impossible or difficult to accurately generate an intended fixation heat in the heating roller. In view of this, in the IH fixing device of the above type, there is provided a positioning mechanism capable of keeping the distance between the induction heating coil and the heating roller unchanged.
There is known a fixing unit provided with a nipping pressure adjusting mechanism configured in such a manner that a nipping pressure in the fixing nip portion is switched between a normal pressurized (high-pressure) state and a depressurized (low-pressure) state. The nipping pressure adjusting mechanism is operable to set the nipping pressure to the normal pressurized state, in the case where a sheet having an ordinary thickness is allowed to pass the fixing nip portion, and is operable to set the nipping pressure to the depressurized state, in the case where thick paper such as an envelope or thin paper is allowed to pass the fixing nip portion, or a jammed sheet is to be removed. A rotary shaft of the pressing roller is supported in the housing of the fixing unit in such a manner that the pressing roller is selectively moved (shifted) in a direction toward or away from the fixing roller, and the nipping pressure adjusting mechanism is operable to switch the nipping pressure state, using a driving force of a motor. The switching of the nipping pressure enables to suppress generation of a crease and the like in passing thick paper or thin paper through the fixing nip portion, or allows the user to easily remove a jammed sheet.
In the conventional apparatus, a pressing roller is used as a roller to be positionally moved in order to easily keep the positional relationship between a fixing roller (a heating roller) and an induction heating coil unchanged. Specifically, if the fixing roller is kept unmoved, the position of the fixing unit relative to the induction heating coil is determined at the point of time when the fixing unit is mounted at a predetermined position of the main body of the image forming apparatus. Thus, there is no likelihood that the position of the fixing unit may be displaced when the fixing nip portion is formed.
Further, in the conventional apparatus, transmission of a rotational driving force from driving means of the apparatus body to each of the rollers in the fixing unit is realized by inputting a driving force to the rotary shaft of the pressing roller. This is because whereas the fixing roller is a sponge-like roller, the pressing roller is a roller having rigidity higher than the rigidity of the fixing roller. In the case where a fixing roller whose outer diameter is likely to vary is used as a roller to which a driving force is inputted, the circumferential speed of the outer circumference of the roller may vary, which makes it impossible or difficult to accurately control the rotation of the roller.
However, as described above, the pressing roller is a roller whose rotary shaft is positionally moved, it is necessary to modify the mechanism for inputting a driving force to the pressing roller. In other words, the above arrangement complicates the mechanism for inputting a driving force to the pressing roller whose axis of rotation is positionally moved, as the nipping pressure state is switched between the normal pressurized state and the depressurized state, and may result in a cost increase.
It is an object of the present disclosure to provide an image forming apparatus of an IH fixing system provided with an IH fixing unit detachably mounted on the apparatus body, which enables to simplify a mechanism for inputting a rotational driving force to the fixing unit.